In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. Active efforts have been made on the development of microfabrication technology using deep ultraviolet and vacuum ultraviolet lithography. The photolithography using a light source in the form of a KrF excimer laser of wavelength 248 nm has reached a level enough to play a main role in the commercial production of semiconductor devices. Investigations are made on the use of an ArF excimer laser of wavelength 193 nm in order to achieve a further reduction of feature size, and the technical challenge has reached the pilot production of some devices. Since the ArF excimer laser lithography has not matured as the micropatterning technology, a number of problems must be overcome before it can enter the commercial phase of device production.
Resist materials adapted for the ArF excimer laser lithography must meet several characteristics including transparency at wavelength 193 nm and dry etch resistance. As the resist material satisfying both the requirements, JP-A 9-73173 and JP-A 9-90637 disclose resist compositions comprising as a base resin a polymer of (meth)acrylic acid derivative having bulky acid-labile protective groups as typified by 2-ethyl-2-adamantyl and 2-methyl-2-adamantyl. While a number of resist materials have been proposed since then, most of them are common in that the resin used has a highly transparent backbone and a carboxylic acid moiety protected with a tertiary alkyl group.
While these resist materials suffer from many problems, a shortage of resolution is regarded a fatal deficiency in forming micro-size patterns. Specifically, tertiary alkyl groups which are often used as the protective group for carboxylic acids are generally less reactive, and their performance is absolutely short in the application where a high resolution is required as in forming fine trenches. Reactivity can be enhanced to some extent by increasing the temperature of heat treatment following exposure. However, the elevated temperature encourages acid diffusion, thereby exacerbating pattern density dependency and mask fidelity. An overall evaluation does not reach the conclusion that the resolution performance is enhanced.
As a protective group having a relatively high reactivity, JP-A 2006-96965 discloses the use of protective groups of methylene acetal type. The use of protective groups of methylene acetal type has advantages including improved sensitivity and enhanced resolution, but suffers from the drawback that light leakage from exposure site has a substantial influence, inviting the undesired results of film slimming and degraded pattern rectangularity.